#162935 added a boolean setting to allow merged platform and UI thread. Using boolean doesn't allow for opting out of the behavior once we enable it by default, which is something we will likely want to allow, at least for a period of time.

This PR replaces the boolean with following enum:

// Configures the thread policy for running the UI isolate.
typedef enum {
  // Default value. Currently will run the UI isolate on separate thread,
  // later will be changed to running the UI isolate on platform thread.
  Default,
  // Run the UI isolate on platform thread.
  RunOnPlatformThread,
  // Run the UI isolate on a separate thread.
  RunOnSeparateThread,
} FlutterDesktopUIThreadPolicy;

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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